This invention is in the field of active filter networks, particularly networks which can be tuned from one predetermined center frequency to another over a range of adjustable possible center frequencies. Such networks are useful, for example, in signal processing systems.
Circuits using operational amplifiers to synthesize bandpass filters have been known for some time. Such a circuit is described in the paper titled "The Biquad: Part I -- Some Practical Design Considerations," Lee C. Thomas, Transactions on Circuit Theory, May 1971, Volume CT-18, of Institute of Electrical and Electronics Engineers.
Circuits using operational amplifiers to synthesize active filter networks having a variable center frequency have also been known for some time. Typical examples are shown in U.S. Pat. Nos. 3,731,218, 3,755,749 and 3,803,429. The primary disadvantage of the circuits of the prior art is that they are only switchable from one predetermined center frequency to a second or plurality of predetermined center frequencies. The prior methods of switching allow circuit transients to be introduced into the filter due to the multiplicity of switches required at each node or the existence of momentary open circuit states.
The prior art active filter networks are switched by a number of switching circuits but none show an active filter network where the center frequency can be controlled by a single common control. As a result, it has not been possible to obtain stable switch tunable bandpass filters in the active filter networks of the prior art.